These are expressions, not equations.
Keep in mind that x by itself is the same as 1x.
x is a variable that stands for a number, but we can use x differently to understand this better.
Think of x as being an object. x means "1 book".
Then, 2x means 2 books, 3x means 3 books, etc.
x alone is the same as 1x, or 1 book.
Translate each problem into numbers of books.
2x + x =
2x + 1x =
2 books + 1 book = 3 books,
so 2x + x = 3x
4x + x = 4x + 1x =
4 books + 1 book = 5 books, so
4x + x = 5x
7x + 3x =
7 books + 3 books = 10 books, so
7x + 3x = 10x